We plan to continue our efforts to elucidate the mechanism whereby the antibiotic pactamycin interferes with the initiation of polypeptide chains in cells from eukaryotes. This will involve studies on the binding of radioactive pactamycin to ribosomes at various stages during protein synthesis and study of the specific step inhibited that leads to the accumulation of inactive initiation complexes in fractionated systems and of initiator dipeptide in reticulocyte lysates. The mechanism of inhibition of the peptidyl transferase by the antibiotic sparsomycin will also be studied in in vitro systems from prokaryotic and eukaryotic cells. We have also begun to study the mechanism whereby the antitumor antibiotic neocarcinostatin interferes with DNA replication in HeLa cells.